Event disambiguation

ABSTRACT

A method of identifying at least two proximate devices ( 1, 2 ) comprises the steps of: recording, in each device, an audio signal (AS) so as to produce respective audio samples, detecting, in each device, a sensory identifier serving as a trigger (TR), comparing the audio samples during a common interval (I) defined relative to the trigger (TR), and deciding that the event is common to the devices only if the respective audio samples are sufficiently similar. The common interval (I) starts prior to the trigger (TR) and preferably also ends prior to the trigger.

The present invention relates to event disambiguation. More inparticular, the present invention relates to a method of and a devicefor identifying proximate devices by using a sensory identifier, thatis, to a method of and a device for pairing at least two devices if theyare involved in a common event.

International Patent Application WO 2009/014438 (TNO/KPN) discloses amethod of identifying proximate mobile devices by using sensoryidentifiers, that is, identifiers which can be perceived with thesenses. Such a sensory identifier may for example be an audioidentifier, such as a particular sound, or a video identifier, such as aparticular image. The identifier serves to (uniquely) identify a commonevent, such as two mobile devices touching or two mobile devicesregistering the same symbol. However, even if an identifier is unique,it may be ambiguous when detected. The sound of two mobile devices beingtapped together may be unique to those devices, but may be reflected andbe detected again as an echo, or may be picked up by other mobiledevices which were not involved in the tapping event. This may lead toidentification errors, as some devices may detect a sensory identifierthey are not proximate to, for example an echo of a touching event.

International Patent Application WO 01/81885 (Metravib) discloses amethod and installation for detecting and locating a source of noisesand vibrations in a pipe system. This known method relates to anentirely different field of technology: locating defects in pipes. Inthis known method, at least two sensors mounted on a pipe are separatedby a known distance, which allows the location of a noise source to bedetermined. A triggering time may be followed by a period of noisesignal analysis. The noise signal captured during this period may betransmitted to neighbouring sensors for intercorrelation in order toincrease the sensitivity of the sensors. WO 01/81885 fails to suggest asolution for the problem of event disambiguation, in particular in thecontext of device pairing.

It is an object of the present invention to overcome these and otherproblems of the Prior Art and to provide a method of identifyingproximate devices using a sensory identifier, in which method the chanceof erroneous identifications is significantly reduced.

It is a further object of the present invention to provide a method ofidentifying proximate devices using a sensory identifier which can evenbe carried out without requiring the content of the sensory identifiers.

It is a still further object of the present invention to provide adevice, preferably a mobile consumer device, as well as a system forcarrying out the method of the present invention.

Accordingly, the present invention provides a method of identifyingproximate devices, the method comprising the steps of:

-   -   recording, in each device, an audio signal so as to produce        respective audio samples,    -   detecting, in each device, a sensory identifier,    -   using said detecting as a trigger and using said sensory        identifier for matching the devices,    -   comparing the audio samples during a common interval defined        relative to the trigger, and    -   deciding that the devices are proximate only if the respective        audio samples are sufficiently similar.        By detecting a sensory identifier, such as a sound or image, by        each device concerned, a matching process can be started: all        devices that detected the same sensory identifier are assumed to        be proximate to each other. In addition, the detection of the        sensory identifier is used as a trigger, creating a reference        point in time. By comparing audio samples of substantially the        same time interval it can be determined whether those audio        samples match. If the audio samples match, that is, if they are        sufficiently similar, it is decided that the event providing the        sensory identifier is common to the devices: their “audio        background” is (nearly) the same and proximity may be assumed.        If the audio samples fail to match, that is, if they are too        dissimilar, it is decided that the event providing the sensory        identifier is not common to the devices and that the sensory        identifier has led to a false detection. In case of an audio        identifier, for example, erroneously detecting the audio        identifier (that is, the sound) may be due to reflections off        walls or ceilings.

It can therefore be seen that the invention substantially reduces oreven eliminates false common proximity detections by taking the “audiobackground” of the devices into account. It will be understood thatdeciding that the devices are proximate, that is, that the devices arein the vicinity of each other, is only possible if the devices concernedhave all detected matching representations of a sensory identifier. Inother words, each device which detected a sensory identifier made arepresentation (i.e. a numerical code) of the detected sensoryidentifier, and the representations of the devices concerned were foundto match, that is, to be identical or to be sufficiently similar. Therepresentations may be exchanged between the devices, or between thedevices and a server, to establish the match.

To determine whether audio samples are sufficiently similar, the audiosamples (that is, the audio recordings) may be aligned (that is, theymay be synchronised in time) and then their (dis)similarity may bedetermined using a suitable measure, for example the least squaresmethod, and subsequently be compared with a threshold value. Thethreshold value may be predetermined but may also be dynamic, that is,adjusted according to the circumstances.

The method of the present invention can be used with two devices, butalso with three or more devices, for example, four, five, eight, ten oreven more. It is noted that the step of detecting a sensory identifiermay comprise detecting the providing of a sensory identifier, and that aprior step of providing a sensory identifier may be included in someembodiments of the present invention.

It is possible to start recording the audio samples when the trigger isdetected. In a preferred embodiment, however, the common interval startsprior to the trigger and preferably also ends prior to the trigger. Thatis, the audio sample is preferably pre-recorded, before the trigger isdetected. This offers the advantage that the determination whether theaudio samples match can be made sooner, as there is no need to wait forthe recording to finish: the recording has already started and haspreferably also finished before the trigger was detected.

The common interval may also overlap with the trigger, however, it ispreferred that the common interval ends before the point in time atwhich the trigger occurs or, at least, is detected.

Advantageously, the method of the present invention may comprise thefurther step of comparing the audio samples during an additional commoninterval also defined relative to the trigger, wherein the additionalcommon interval starts after the trigger. By using two time intervals,one before and one after the trigger, a more reliable determination ofthe “audio background” can be made, at the expense of a slightly longerrecording and/or processing time.

In a preferred embodiment, the sensory identifier is part of the audiosignal. That is, the sensory identifier is detected by analysing theaudio signal and searching for a peak, trough, pattern, or otherdetectably signal property, which detection is then used as a trigger.

The audio samples may consist of series of digitised values of the audiosignals. However, the amount of data to be stored and/or processed maybe significantly reduced if the audio samples are represented bycharacteristic parameters. It is well known to reduce audio samples tosets of parameters, such as average amplitude, average frequency,bandwidth, and other parameters.

In a first embodiment, the step of comparing is carried out in a server.This requires the transmission of the audio samples to the server andthe transmission of the matching results back to the mobile devices, butreduces the required amount of processing in the mobile devices. In asecond embodiment, the step of comparing is carried out in at least oneof the devices. It is possible for the comparing step to be carried outin both devices and then to exchange the results, which provides anextra check. However, the comparing step may also be carried out in asingle device.

The step of using the sensory identifier for matching the devices mayadvantageously comprise exchanging messages between the devices, atleast some of said messages containing a representation of the sensoryidentifier. In addition, the step of using the sensory identifier formatching the devices may advantageously comprise comparingrepresentations of the sensory identifier and deciding that a matchexists if the representations are sufficiently similar.

Accordingly, a first device may send a message containing arepresentation of the sensory identifier to a second device, which thencompares its representation with the representation contained in thereceived message and sends an acknowledgement is a match is made.Alternatively, a first and a second device may each send a messagecontaining their respective representation of the sensory identifier toa server, which compares the representations and sends anacknowledgement to each device if a match is made.

In a particularly preferred embodiment, the trigger is produced bytapping the two devices together. The sound of the two devices touchingserves as a sensory (i.e. audio) identifier for identifying the tappingevent.

The devices may be mobile consumer devices, for example mobile telephonedevices, which are preferably arranged for playing games. However, thepresent invention is not limited to mobile devices and at least one ofthe devices may be stationary.

The present invention also provides a computer program product forcarrying out the method as defined above. A computer program product maycomprise a set of computer executable instructions stored on a datacarrier, such as a CD or a DVD. The set of computer executableinstructions, which allow a programmable computer to carry out themethod as defined above, may also be available for downloading from aremote server, for example via the Internet.

The present invention additionally provides a device for identifyingproximate devices, the device comprising:

-   -   a recording unit for recording an audio signal so as to produce        respective audio samples,    -   a detection unit for detecting a sensory identifier and for        producing a trigger in response thereto,    -   a matching unit for matching representations of sensory        identifiers,    -   a comparison unit for comparing the audio samples during a        common interval defined relative to the trigger, and    -   a decision unit for deciding that the devices are proximate only        if the respective audio samples are sufficiently similar.        The device of the present invention has the same advantages as        the method of the present invention. The device for identifying        proximate devices may itself be one of the proximate devices, or        may be a remote device such as a remote server.        In a preferred embodiment, the common interval starts prior to        the trigger and preferably also ends prior to the trigger.        Advantageously, the comparison unit may be further arranged for        comparing the audio samples during an additional common interval        also defined relative to the trigger, wherein the additional        common interval starts after the trigger.

The device of the present invention may be a mobile consumer device, forexample a mobile telephone device, and is preferably arranged forplaying games. A suitable game is Living Stratego®, although theinvention is not limited to this particular game. The present inventionfurther provides a system for gaming comprising at least one device asdefined above.

The present invention still further provides a server for carrying outthe method of the present invention. Such a server is preferablyarranged for receiving audio samples from devices and comprises acomparison unit for comparing the audio samples during a common intervaldefined relative to the trigger, and a decision unit for deciding thatthe event is common to the devices if the respective audio samples aresufficiently similar.

The present invention will further be explained below with reference toexemplary embodiments illustrated in the accompanying drawings, inwhich:

FIG. 1 schematically shows exemplary embodiments of portable consumerdevices for use in the method of the present invention.

FIG. 2 schematically shows an exemplary audio signal and suitable timeintervals as used in the present invention.

FIG. 3 schematically shows a flow diagram representing an embodiment ofthe method according to the present invention.

FIG. 4 schematically shows an embodiment of a audio fingerprintarrangement according to the present invention.

FIG. 5 schematically shows a microprocessor arrangement for use in thepresent invention.

The mobile telephone devices 1 and 2 shown merely by way of non-limitingexample in FIG. 1 each comprise a loudspeaker 17 and a microphone 18.The mobile telephone devices 1 and 2 are examples of portable consumerdevices which can be “matched” when they are in each other's proximity.This “matching” or “pairing”, which may for example be achieved bytapping the devices together, may be used to exchange informationbetween the devices. This information may comprise telephone numbers orother identification information, which in turn can be used to playgames and/or engage in other activities. Thus, tapping the mobiletelephone devices together may constitute an invitation to play a gameor to share information. Reference is made to International PatentApplication WO 2009/014438 mentioned above, which explains such pairingprocesses in more detail.

When the mobile telephone devices 1 and 2 are tapped together, theresulting tapping sound is received substantially simultaneously by themicrophones 18 of both mobile devices. This tapping sound can be used asan identifier for identifying the pairing attempt: it is very likelythat only devices which detect the tapping sound are involved in theencounter. It can thus be said that the tapping sound is an audioidentifier, a special case of a sensory identifier: an identifier thatcan be perceived by the (human) senses. However, other identifiers mayalso be used for pairing, for example pressing buttons more or lesssimultaneously. The present invention is, however, primarily concernedwith (but not limited to) audio identifiers.

The audio identifier may alternatively be constituted by a sound signalproduced by a loudspeaker (e.g. a loudspeaker 17), the sound signalpreferably consisting of multiple tones and/or pulses so as to be ableto distinguish a plurality of different identifiers and/or time stamps.Such sound signals may in turn be produced in response to pressing abutton.

When using sensory identifiers, in particular audio identifiers, forpairing or similar actions the problem of false matches arises. Themicrophones of the mobile devices may pick up reflections of the(tapping or other) sound off walls, and also sounds (or theirreflections) made by other devices. In practice, therefore, the sound(or a similar identifier) is often not sufficient to properly identifythe actual devices involved.

For this reason, the present invention proposes to use additionalinformation for identifying the devices involved, and in particular, touse the “audio background” of the devices. To this end, the presentinvention proposes to receive and record an audio signal in both (orall) devices, and to compare the audio signals (or theirrepresentations). If the audio signals are sufficiently similar, theaudio backgrounds (or acoustic backgrounds) of the devices match and itcan be concluded that the devices were involved in the same encounter,in particular if the sensory identifiers also match. Accordingly, thematching of the audio signals can be used as a check of the matching ofthe sensory identifiers: if both the sensory identifiers and the audiosignals match, the proximity of the devices is established with muchgreater certainty.

The (detection of the) audio identifier, such as the tapping sound, isin the present invention used as a trigger for triggering the comparisonprocess. It is preferably also used as an identifier, for example bydetermining its properties (e.g. time of occurrence, amplitude and/orpower, duration, etc.), for identifying the event which produced theaudio identifier.

The inventive process of determining the audio background of a triggeris illustrated in FIG. 2, where the amplitude A of an audio signal AS isshown as a function of the time t. The audio signal AS may be receivedby the microphones 18 of the mobile telephone devices 1 and 2 of FIG. 1,but may also be received by a third device. The audio signal AS isrecorded, that is, is received, stored and/or converted into a suitablerepresentation (such as a digital representation after analogue/digitalconversion, optionally followed by parameter extraction). The amplitudeA, which may be measured in volts (V), is shown to have an initialaverage value A₀, while deviations from this value A₀ are mainly due tobackground noise. At t₃, the audio signal amplitude reaches a peak P,indicative of an audio identifier, such as the tapping together of twomobile telephone devices. The detection of the peak P triggers acomparison of the background noise during certain time intervals. Forthis reason, the peak P may be considered a trigger TR.

It is noted that a similar type of background matching may be carriedout for video images, in particular when the sensory identifier is avisual identifier. A “video background” or “image background” may bematched to increase the reliability of matching on the basis of a visualidentifier. It is noted that audio backgrounds may also be used toenhance the reliability of visual identifiers.

In FIG. 2, two background noise measuring intervals are shown by way ofnon-limiting example. A first interval I extends from t₁ to t₂, andtherefore both begins and ends before the occurrence of the trigger P,while a second interval II extends from t₄ to t₅ and therefore bothbegins and ends after the occurrence of the trigger P. Other intervalsmay be used (not shown) which may include the peak P. The audio signalsAS, or their representations (that is, samples), recorded by both mobiletelephone devices (1 and 2 in FIG. 1) are compared during at least oneinterval. If the audio signals of the two devices during the interval(s)are sufficiently similar, then it may be concluded that the devicesshare an acoustic background and that they are in each other'sproximity. In other words, it may be concluded that the event causingthe trigger (here: the peak P in the audio signal) was common to thedevices.

It will be understood that the number of audio signals that may becompared is not limited to two, and that the audio signals from three,four or even more devices may be compared.

The audio signals may be compared in the first interval I, in the secondinterval II or in both. In a preferred embodiment of the presentinvention, the audio signals are only compared in the first interval I,prior to the trigger TR. This offers the advantage of being able tostart comparing the signals immediately after the trigger, withouthaving to wait for a suitable recording interval to finish. Thisembodiment requires the audio signal to be recorded prior to thetrigger, a continuous recording is therefore preferred. It will beunderstood that continuous recording is not required when only intervalII is used, since then the recording may be started by the trigger.

The length of the interval(s) may depend on the particular applicationand may be dynamically adjustable. In practice, the length of aninterval may range from a few ms (milliseconds) to approximately one s(second). The preferred gap between t₂ (end of the first interval) andt₃ (trigger) may range from a few ms to a few hundred ms.

A preferred embodiment of the present invention is summarised in theflow diagram of FIG. 3. In step 30, the method starts. In step 31, theaudio signal (AS in FIG. 2) is recorded, resulting in audio samples. Asnoted above, this recording may take place continuously as long as itmay be necessary to identify events.

In step 32, a trigger is detected, such as an audio identifier (peak Pin FIG. 2). This leads to step 33, in which audio samples are compared.The audio samples (or audio fragments) are stored audio signals or theirrepresentations. The audio samples may originate from two or moredevices. In step 34, it is decided whether the audio samples aresufficiently similar. If they are, the event is common to the devicesand a signal indicating this is produced in step 35, after which theroutine returns to step 32 (or 31 if the audio signals are not recordedcontinuously). If the audio samples are not sufficiently similar, theevent is not is common to the devices and a signal indicating this isproduced in step 36, after which the routine also returns to step 32 (or31 if the audio signals are not recorded continuously).

The extent of the similarity of the audio samples may be determinedusing a known method and criterion. For example, the audio samples maybe aligned (in time), their mean values may be reduced to zero, andtheir absolute amplitudes may be established. Then the mean square error(MSE) may be determined using the difference of the absolute amplitudes.Subsequently, the resulting mean square error may be compared with athreshold value. This threshold value may be predetermined or may beadjusted dynamically.

With regard to the time intervals during which audio samples arecompared, there are several possibilities, which each offer specificadvantages.

In a first embodiment, the audio signals are only recorded during thetime interval. In the example of FIG. 2, the recording may thus takeplace from t₄ to t₅, during time interval II. The start of this timeinterval, at t₄, may take place as soon as possible after the detectionof the trigger. In this embodiment, therefore, the recording only takesplace during the interval and then substantially the entire timeinterval is used in the comparison. This saves memory space but onlyallows time intervals after the trigger.

In a second embodiment, the audio signals are recorded virtuallycontinuously and a section (that is, audio sample) of these recordingsis used for the comparison (in some embodiments, more than one sectionmay be used). In this embodiment, the time intervals correspond with theselected sections. This embodiment requires more memory space but allowsaudio samples recorded prior to the trigger to be used.

As stated above, the audio signals are represented by audio samples,which may be digitised audio fragments, or their representations, forexample by characteristic parameters. Suitable parameters are averageamplitude, time of occurrence, average frequency, bandwidth, and otherparameters. By producing suitable characteristic parameters, a“fingerprint” of each audio signal can be made. This will further beexplained with reference to FIG. 4.

The merely exemplary arrangement 4 of FIG. 4 comprises a microphone 41,a touch event detector (TED) 42, a background noise recorder (BNR) 43,and a fingerprint determination unit (FPD) 44. The arrangement 4 isdesigned for detecting “touch events”, that is, events in which two(mobile) devices touch, for example when being tapped together so as toinitiate a matching process.

The microphone 41 receives sound and produces a microphone signal MS(equivalent to the audio signal in FIG. 2), which is fed to both thetouch event detector 42 and the background noise recorder 43. The touchevent detector 42 detects a peak (P in FIG. 2) in the microphone signal,which peak is indicative of a touch event. The detection may be a simplethreshold detection, or any other suitable detection. In response to thedetection of the peak, the touch event detector 42 produces a triggersignal TR, which is fed to the fingerprint determination unit 44. Signalcharacteristics of the peak may additionally be used as an audioidentifier for matching proximate devices.

The background noise recorder 43 records the microphone signal for thepurpose of registering the background noise (or, more in general,background sound) and passes it on to the fingerprint determination unit44. When triggered by the trigger signal TR, the unit 44 produces afingerprint of the background noise signal by extracting suitableparameters of the signal. This fingerprint FP is then passed on to anevent server or other suitable apparatus, such as the other of the twodevices shown to be touching in FIG. 1. The communication with theserver is preferably wireless, using an appropriate communication link(Bluetooth®, Ethernet, GMS, UMTS, GPRS, HSDPA, . . . ).

The arrangement of FIG. 4 may be implemented in hardware, in software,or in a combination of hardware and software.

The background noise or background sound constituting the “audiobackground” of the devices may, in part, be generated by one or all ofthe devices involved. At least one of the devices could, using itsloudspeaker (17 in FIG. 1), produce an (audible and/or ultrasonic) audiosignal in order to contribute to a common audio background. The audiosignal could consist of a single tone, but could also comprise a certainsound pattern. The production of an audio signal by the devices could besubstantially continuous, but could also be initiated by the trigger.When two or more devices produce an audio signal, they could do soeither simultaneously or in turns. In the latter case the devices couldbe arranged for “listening” to other, similar devices and producing anaudio signal in response to a detected audio signal.

An exemplary embodiment of a device for carrying out the method of thepresent invention is schematically illustrated in FIG. 5, where thedevice 5 is shown to comprise a microprocessor unit 51, a memory unit 52and an input/output (I/O) unit 53. The memory unit 52 may comprisesuitable software programs for making the microprocessor unit 51 carryout the steps of the present invention, such as detecting a trigger,comparing audio samples and/or deciding whether an event is common tothe devices.

The device 5 of FIG. 5 may be a server, arranged for receiving the audiosignals from a plurality of portable consumer devices, such as themobile telephone devices shown in FIG. 1, a laptop computer, a PDA(personal digital assistant), or an MP4 player. Alternatively, thedevice 5 may be incorporated in a portable consumer device.

Although the present invention has been explained with reference topairs (or triplets, or quadruplets) of mobile devices, it is notessential that all devices involved are mobile. The present inventioncan also be carried out with a mobile (or at least portable) device anda stationary device.

The present invention is based upon the insight that the acousticbackground of an acoustic identifier may assist in reducing the numberof false or erroneous detections of proximate devices, in particular byadding a check to the identification process. The present inventionbenefits from the further insight that determining the acousticbackground in a time interval prior to the acoustic identifier shortensthe detection time.

The sensory identifier (an identifier that is perceptible by the humansenses) is detected from an audio sensing signal or a sensing signal foranother type of sense such as touch (contact) sensing, image sensing,sensing that a button is pushed etc. The detections of the sensoryidentifiers of different devices may be performed independently for eachof the devices, for example in each device, by detecting signal peaks,signal troughs or signal patterns. These detections are used for twopurposes: to perform matching with sensory identifiers of other devicesand as a trigger to select a time interval of audio samples atpredetermined relative temporal position with respect to a time point ofthe detected sensory identifier.

When the matching of the detected sensory identifiers produces theidentification of a pair or group of devices with matching sensoryidentifiers, a comparison between the audio samples of the devices isused, for the time intervals that were selected by the triggers of thosedevices. If the audio in these time intervals also matches, it isdecided that the devices are proximate and actions are enabled thatdepend on this decision. As a result, different forms of comparison maybe used for matching sensory identifiers and for comparing audio in theselected time intervals respectively. The comparison of audio in theselected time intervals can be performed without first detectingselected identifiers in the audio. A more time consuming type ofcomparison, using significantly more data, may be used for comparison ofaudio in the selected time intervals than for finding matchingidentifiers. A more robust type of comparison may be used that involvessuppression of incidental differences such differences between thedevices in amplitude sensitivity or distribution of spectralsensitivity.

It is noted that any terms used in this document should not be construedso as to limit the scope of the present invention. In particular, thewords “comprise(s)” and “comprising” are not meant to exclude anyelements not specifically stated. Single (circuit) elements may besubstituted with multiple (circuit) elements or with their equivalents.

It will be understood by those skilled in the art that the presentinvention is not limited to the embodiments illustrated above and thatmany modifications and additions may be made without departing from thescope of the invention as defined in the appending claims.

1. A method of identifying proximate devices, the method comprising thesteps of: recording, in each device, an audio signal so as to producerespective audio samples, detecting, in each device, a sensoryidentifier, using said detecting as a trigger and using said sensoryidentifier for matching the devices, comparing the audio samples duringa common interval defined relative to the trigger, and deciding that thedevices are proximate only if the respective audio samples aresufficiently similar.
 2. The method according to claim 1, wherein thecommon interval starts prior to the trigger.
 3. The method according toclaim 1, wherein the common interval ends prior to the trigger.
 4. Themethod according to claim 1, comprising the further step of comparingthe audio samples during an additional common interval also definedrelative to the trigger, wherein the additional common interval startsafter the trigger.
 5. The method according to claim 1, wherein thesensory identifier is part of the audio signal.
 6. The method accordingto claim 1, wherein the audio samples are represented by characteristicparameters.
 7. The method according to claim 1, wherein the step ofcomparing the audio samples is carried out in a server.
 8. The methodaccording to claim 1, wherein the step of comparing the audio samples iscarried out in at least one of the devices.
 9. The method according toclaim 1, wherein the step of using said sensory identifier for matchingthe devices comprises exchanging messages between the devices, at leastsome of said messages containing a representation of the sensoryidentifier.
 10. The method according to claim 1, wherein the step ofusing said sensory identifier for matching the devices comprisescomparing representations of the sensory identifier and deciding that amatch exists if the representations are sufficiently similar.
 11. Themethod according to claim 1, wherein the sensory identifier is producedby tapping the devices together.
 12. The method according to claim 1,wherein the devices are mobile consumer devices, for example mobiletelephone devices.
 13. The method according to claim 12, wherein thedevices are arranged for playing games.
 14. A computer program productfor carrying out the method according to claim
 1. 15. A device foridentifying proximate devices, the device comprising: a recording unitfor recording an audio signal so as to produce respective audio samples,a detection unit for detecting a sensory identifier and for producing atrigger in response thereto, a matching unit for matchingrepresentations of sensory identifiers, a comparison unit for comparingthe audio samples during a common interval defined relative to thetrigger, and a decision unit for deciding that the devices are proximateonly if the respective audio samples are sufficiently similar.
 16. Thedevice according to claim 15, wherein the comparison unit is arrangedsuch that the common interval starts prior to the trigger.
 17. Thedevice according to claim 15, wherein the comparison unit is furtherarranged for comparing the audio samples during an additional commoninterval which starts after the trigger.
 18. The device according toclaim 15, further arranged for producing an audio signal, preferably inresponse to the trigger.
 19. The device according to claim 18, whereinthe audio signal produced is at least in part ultrasonic.
 20. The deviceaccording to claim 15, which is a mobile consumer device, for example amobile telephone device.
 21. The device according to claim 20, whichdevice is arranged for playing games.
 22. A system for gaming,comprising a device according to claim 15.